Cemented tungsten carbide, such as WC--Co is well known for its mechanical properties of hardness, toughness and wear resistance, making it a popular material of choice for use in such industrial applications as mining and drilling where its mechanical properties are highly desired. Because of its desired properties, cemented tungsten carbide has been the dominant material used as cutting tools for machining, hard facing, wear inserts, and cutting inserts in rotary cone rock bits, and substrate bodies for drag bit shear cutters. The mechanical properties associated with cemented tungsten carbide and other cermets, especially the unique combination of hardness toughness and wear resistance, make these materials more desirable than either metals or ceramics alone.
For conventional cemented tungsten carbide, fracture toughness is inversely proportional to hardness, and wear resistance is proportional to hardness. Although the fracture toughness of cemented tungsten carbide has been somewhat improved over the years, it is still a limiting factor in demanding industrial applications such as high penetration drilling, where cemented tungsten carbide inserts often exhibit gross brittle fracture that leads to catastrophic failure. Traditional metallurgical methods for enhancing fracture toughness, such as grain size refinement, cobalt content optimization, and strengthening agents, have been substantially exhausted with respect to conventional cemented tungsten carbide. The mechanical properties of commercial grade cemented tungsten carbide can be varied within a particular envelope by adjusting cobalt metal content and grain sizes. For example, the Rockwell A hardness of cemented tungsten carbide can be varied from about 85 to 94, and the fracture toughness can be varied from about 8 to 19 ksi.in.sup.-1/2. Applications of cemented tungsten carbide are limited to this envelope.
Another class of materials for cutting and wear applications is tool steel. In general, the wear resistance of steels, including tool steel, is much lower than that of cemented tungsten carbide. U.S. Pat. No. 5,290,507 describes a material that is formed by incorporating a certain percentage of cemented tungsten carbide granules into a matrix of tool steel binder to increase the wear resistance of the tool steel. Such tool steel/cemented tungsten carbide composite materials belong to the category of metal matrix composites, where the brittle phase, i.e., cemented tungsten carbide granules, is the minority phase.
A problem known to exist with tool steel/cemented tungsten carbide composites is that iron (Fe) present in the tool steel binder tends to react with the cemented tungsten carbide to form Fe.sub.3 C, which can be detrimental to the ductility and toughness of the composite. For this reason, such tool steel/cemented tungsten carbide composites are not desired for use in applications, such as those discussed above, where improved toughness is needed. Additionally, the limited ductility of the tool steel that is used to form the cemented tungsten carbide composite also acts to limit the overall toughness of the composite, thereby limiting its use.
It is, therefore, desirable that a cemented tungsten carbide composite be developed that has improved properties of fracture toughness when compared to conventional cemented tungsten carbide materials. It is desirable that such cemented tungsten carbide composite have such improved fracture toughness without sacrificing wear resistance, i.e., having equal or better wear resistance than that of conventional cemented tungsten carbide materials. It is desired that such cemented tungsten carbide composites be adapted for use in such applications as roller cone bits, percussion or hammer bits and drag bits, and other applications such as mining and construction tools where properties of improved fracture toughness is desired.